Bile acids and their salts have a complex physiological function in lipid absorption and the formation and dissolution of gallstones. This function can be correlated in a qualitative manner with their detergent- like properties. Like other hydrophobic solutes, bile salts are capable of hydrophobic bonding. They self-associate, and form mixed aggregates with a variety of lipids. Recent research by the author and others has revealed the importance of the molecular structure of hydrophobic solutes in producing different patterns of association, and micellar solubilization. The main objective of this research is to investigate the role of the unique structure of bile salts, and to what extent these physiological surfactants differ from ordinary detergents. The customary assumption that they behave like micelle-forming surfactants is crude and often misleading. Attempts are being made to adapt two partition methods developed by the principal investigator for studying the self-association of fatty acid anions and ionic dyes for investigating bile salts. A simple, sensitive, analytical method for the taurodeoxycholate anion has been developed and used to study the extractivity of taurodeoxycholate into chloroform using several counterions. Optical rotatory dispersions of several bile salts and the circular dichroism of sodium dehydrocholate have been investigated. The solubilization characteristics of the sulfonphthalein type indicator dye bromothymol blue in two bile salts and the classical detergents sodium alkyl sulfates have been compared.